Recently, an ultrasonic diagnostic apparatus has been put to practical use, which acquires echo signals from an object by three-dimensionally scanning the object. This ultrasonic diagnostic apparatus can generate and display a three-dimensional image (e.g., a rendering image) by generating three-dimensional B-mode data based on an echo signal.
There is available an imaging method (to be referred as a cavity imaging method hereinafter) which three-dimensionally displays a lumen region (e.g., a blood vessel or biliary duct) in a scanned region with high luminance. An image generated by the cavity imaging method will be called a cavity image hereinafter. A cavity image is generated by reversing the gradations of three-dimensional B-mode data and then generating a three-dimensional image (e.g., a rendering image).
Gradation reversal will be described below. Assume that the gradation values of B-mode data range from 0 to 255, and the gradation value of given B-mode data is 10. In this case, reversing the gradation is to change the gradation value of the B-mode data to 255−10=245. A gradation value corresponds to a luminance value. If, therefore, the gradation value of the B-mode data is a low gradation like 10, it corresponds to a low luminance. If the gradation value of the B-mode data is a high gradation like 245, it corresponds to a high luminance.
The cavity imaging method is designed to reverse the gradations of a plurality of signal values or a plurality of pixel values of three-dimensional B-mode data. Performing gradation reversal will change the gradation of virtual data concerning a non-lumen region (e.g., a parenchyma organ or the like in the object) with a high gradation (high luminance) to a low gradation (low luminance). This operation also changes the gradations of lumen data concerning the lumen region with low gradations (low luminances) or in a transparent state to high gradations (high luminances). With this operation, the lumen region is displayed. Note that in this state, since virtual data lower in gradation than the lumen region exist outside the high-gradation lumen region, non-lumen region may be displayed so as to surround the lumen region, as shown in (a) in FIG. 17. In this case, it is difficult to recognize the lumen region. At this time, setting a threshold and removing virtual data lower in gradation than the threshold can display only the high-gradation lumen region. For the sake of printing, (a) in FIG. 17 shows a black/white reversed image. The same applies to the images shown in (b) in FIG. 17, (c) in FIG. 17, (a) in FIG. 18, and (b) in FIG. 18.
In practice, however, since echo signals are attenuated due to insufficient contact between an object surface and the ultrasonic probe or the attenuation of the intensity of ultrasonic waves at deep portions in a scanned region, mainly the virtual data at side and deep portions in the scanned region have low gradations. In a gradation-reversed cavity image, therefore, the virtual data at side and deep portions have high gradations like the lumen region. For this reason, as indicated by (b) in FIG. 17, threshold processing cannot remove virtual data. The virtual data which are not removed by threshold processing become artifacts in the cavity image. These artifacts decrease the detection performance and diagnostic performance concerning the lumen region by the operator. At this time, increasing the threshold to eliminate artifacts will simultaneously make the lumen region disappear as indicated by (c) in FIG. 17. It is therefore difficult to avoid the influences of artifacts by threshold processing.
The above method of reducing artifacts is a method using a volume removal function. In this method, first of all, the apparatus sets, on a displayed cavity image, the region input by the operator via a trackball. The apparatus then removes a volume image concerning the set region in response to switching operation on the panel of an input unit. The apparatus repeats the above procedure several to ten several times while rotating a cavity image. The above procedure eliminates artifacts. Since this method makes the operator perform input operation to set a region from which artifacts are removed, some artifacts remain unremoved. For this reason, the detection performance and diagnostic performance concerning the lumen region decrease. In addition, since it takes labor and time to set the above region, the method is difficult to execute at the stage of examination and hence is not practical.
In addition, although the initial value of the threshold is set in advance to a value regarded as proper, the optimal threshold generally differs depending on the object and diagnostic region. It is therefore difficult to always set the threshold to the optimal value in an initial state. For this reason, as indicated by (a) in FIG. 18, in an initial state, the apparatus generally displays an image in which many non-lumen regions such as parenchyma organs remain or an inappropriate image in which many parts of the lumen region are lost. At this time, it is necessary to change the threshold to a proper value by making the operator operate a knob or slider on the panel. In addition, when the operator changes the gain value by using a B-mode gain knob to adjust the luminance of a B-mode image, the apparatus also changes the gradation values of the volume image, as indicated by (b) in FIG. 18. This makes it necessary to re-set the threshold. That is, it is necessary to change the threshold setting every time the diagnostic region and gain value concerning a B-mode image are changed. This leads to poor operability and hence to poor examination efficiency.